The total spend for the earthquake rebuild could grow to $50 billion, another $10b above the official estimate, says Canterbury Employers' Chamber of Commerce boss Peter Townsend.

Townsend says "betterment", or people adding extra capital into the rebuild process, is the main reason the cost of the rebuild will jump from the $40b figure cited by Treasury.

Townsend was an early proponent of the $40b figure when the official estimate was $30b in the early part of 2013, but in time others including Prime Minister John Key accepted $40b was closer to the mark.

Townsend, chief executive of the Canterbury chamber, yesterday said he accepted $40b was a good estimate of the repair costs for the 2010 and 2011 Canterbury earthquakes, but estimated $10b extra was being put into upgrading residential and commercial rebuild projects.

"The cost of the damage of the earthquake is about $40 billion, that's probably pretty close," Townsend said. "But that is not the cost of the rebuild. So the cost of the rebuild is whatever had to be rebuilt and it will invariably involve betterment."

For example, there was an estimated $20b of housing damage, but many homeowners were making substantial changes as that damage was repaired.

That extra capital spend on kitchens bathrooms or bedrooms could add another $3b-$4b to residential work alone.

"Everyone I talk to that is repairing their house or rebuilding their house is putting more capital in than the insurance company, because they're putting betterment in. "So you have $200,000 worth of damage to your house you're going to put a new kitchen in, you're going to put a new bathroom in for another $40,000-$50,000 and do above and beyond what the like-for-like replacement of your insurance policy will allow you to do."

With regard to commercial work, Lyttelton Port of Christchurch chief executive Peter Davie said the port was putting about $10m-$12m extra of its own financial resources into the rebuild of the Cashin Quay 2 wharf.

"We are building a better wharf than we had before . . . the reason we do that is we will end up with the most resilient wharf in the country.

"We are building a wharf that is very, very strong and will stay there through future seismic events."

Registered Master Builders chief executive Warwick Quinn said it was obvious that builders were being asked to do extra work for homeowners on top of the insurance repair process.

"People are taking the opportunity to do things that they otherwise would not have done simply because of technical refurbishments . . . They are paying the extra costs associated with maybe insulation or doing bits and pieces over and above what's already there."

Barry Akers, a spokesman for Fletcher EQR, said while extra work might be being done in the residential repair process, the Earthquake Commission technically did not want betterment to take place at the same time as repairs, but separately.

Townsend said those rebuilding in the commercial sector were "invariably" putting in more than the insurance proceeds to these projects. The new buildings were including new energy systems, strengthening and new building techniques. He acknowledged the $50b was a ballpark figure but both betterment and inflation or cost escalation would contribute. Those owners who had decided on a cash settlement might well have to cover for escalation, Townsend said.